1. Field of the Invention
This invention relates to the field of cables. More specifically, the invention comprises a manufacturing method which allows the automation of previously manual processes, as well as providing a superior end product.
2. Description of the Related Art
Ropes and cables have been traditionally made of natural fibers or metals. These would include wire rope and hemp “hawsers.” In the last half century, synthetic fibers such as NYLON and polyester have been commonly used to replace natural fibers. NYLON and polyester ropes have been used for low tension applications, whereas wire ropes (typically steel) have been used for high tension applications.
Those skilled in the art will know that wire rope behaves very differently from NYLON or polyester rope. Though it is stronger for a given cross section, it is also very stiff. If a wire rope is kinked, the constituent strands may undergo plastic deformation—permanently damaging the rope. Thus, wire ropes must be fed around large radius pulleys, pins, or capstans and must otherwise be protected from kinking. Those skilled in the art will also know that wire rope is susceptible to corrosion, typically rusting, as well as fatigue. It is also quite heavy. Thus, there has long been the desire to obtain the high-strength properties of wire rope without its inherent stiffness and weight.
In recent years, progress has been made in this regard. Advanced synthetic fibers have come onto the market in addition to the older synthetic fibers. These fibers include KEVLAR, VECTRAN, ZYLON, and SPECTRA, to name a few. These advanced synthetic fibers typically have smaller diameters for the individual fibers that was common for the older materials—though this is not always the case.
FIG. 1 shows a stranded cable 10. It includes a stranded core 14, composed of a cluster of parallel strands. Surrounding stranded core 14 is braided strand jacket 12. Although the individual strands are depicted as being substantial in size, the reader should appreciate that they are often very small—having the diameter of a human hair or even less. Within the cabling industry, the individual constituents within a bundled cable are called “fibers” or “strands.” Some publications use “strands” for metal constituents and “fibers” for non-metallic constituents. Throughout this disclosure, the term “strands” will be used to describe both metallic and non-metallic constituents.
Braided strand jacket 12 is used, among other purposes, to contain the parallel strands within strand core 14. In its absence, stranded core 14 can become disorganized. Those skilled in the art will know that the style of weave shown in FIG. 1 is one of dozens in common use. In some cases, the entire cross section of a cable is formed by sets of helical wrappings as shown for braided strand jacket 12.
In order to be useful, any cable must be terminated. That is, any cable must have attachments provided on its two ends (or sometimes at one or more intermediate points) to transmit a tensile load. In the simplest form, these terminations can simply be tying the cable to attachment cleats by knotting or looping. In order to maximize the strength of the completed assembly, however, more efficient terminations are desirable. Such terminations can also facilitate connection of the cable via threads, grommets, eyes, snapping features, etc.
FIG. 2 shows one type of termination. Anchor 16 has been affixed to the end of stranded cable 10. Anchor 16 incorporates loading flange 20. In operation, anchor 16 would be passed into a grommet, so that loading flange 20 can bear against a mating surface and place tension on the cable. Numerous other types of terminations are known, including hooks, threaded fasteners, eye fittings, button studs, clevis fittings, etc.
FIG. 3 shows stranded cable 10 from the top and from the side, with the side view having a partial section to reveal the internal details of this particular type of termination. The reader will observe that tapered bore 18 passes completely through anchor 16. One method of locking the termination in place is as follows: Strands 22 of stranded cable 10 are splayed within tapered bore 18. Potting compound 24—typically high strength resin—is then poured into tapered bore 18. Once potting compound 24 solidifies, strands 22 are locked within tapered bore 18.
Of course, potting is not the only method used to affix terminations. The present invention applies to both potted and non-potted terminations. Other methods of affixing terminations include compression fittings and mechanically swaged fittings. However, as potting is a quite common method, most of the examples are presented for use with the potting approach.
Potting is carried out in a variety of ways. Where the use of very fine strands makes wetting difficult, it may be desirable to wet the strands while they are exposed and loose. Anchor 16 is then pulled over the wet strands. The liquid potting compound is then allowed to harden. Whatever potting method is used, the result after hardening is shown in FIG. 3. Loading flange 20 may then be used to put tension on stranded cable 10 without pulling the strands free of anchor 16. FIG. 4 shows the result with anchor 16 removed for visual clarity. The tapered shape of potted region 32 locks it in place. Although a conical shape for tapered bore 18 has been illustrated, many different shapes can be used, so long as they generally expand from left to right in the view as shown, or have ribs or other features which will mechanically engage the potted strands.
Those skilled in the art will know that the alignment of the strands in the finished termination is important to the ultimate strength, stiffness, fatigue resistance, and working life of the stranded cable. The typical goal is to maximize the maximum allowable stress of the completed assembly as a percentage of the maximum allowable stress of one of the individual strands (In other words, the load should be evenly distributed among the strands).
One factor influencing the result is the nature of the transition from the freely flexing portion of the cable to the region locked within a termination (In this case—potted region 32). FIG. 5 shows this transition as termination plane 34. Termination plane 34 is ideally perfectly orthogonal to the central axis of stranded cable 10, meaning that the anchor attached is perfectly parallel to the central axis of stranded cable 10.
Prior art techniques for creating a terminated cable often manage to generally align the termination with the bundle of strands. However, because the strands are free to shift around, it is often true that some strands have more slack along their length than others. Once the terminations are attached, the result is that some strands within the stranded cable are slightly longer then others. When the cable is loaded in tension, the load will be initially transferred to the shortest strands. The shorter strands can overload and break. This causes a cascading failure as the load is carried by fewer and fewer strands. Thus, the load-carrying ability of the stranded cable will be compromised.
The individual strands within stranded cable 10 are also often difficult to process. Although they can be cut using a metal blade, many strands are not cut easily (NYLON and polyester being notable exceptions). The cable as a whole will tend to smash flat or flare out under the cutting knife and deform. Thus, traditional cutting methods may produce varied results. Cutting and terminating processes on stranded cables are also made difficult by the following additional factors:
1. The strands along the length of the cable can easily shift or snag during production processes, thereby causing misalignments;
2. The accurate positioning and application of a temporary localized binding mechanism (such as tape or bundling string) is difficult (explained in more detail subsequently). The length between the binding mechanism and the end of the cable is critical for properly creating the termination. Thus, the inability to accurately position the binding mechanism introduces error;
3. The addition of a localized binding mechanism causes an increased overall diameter, which may snag on machinery, feed holes, terminations, or the like;
4. A temporary localized binding mechanism is generally unable to bind the strands tightly. It is also difficult to maintain a predictable outer diameter. This fact means that bores through the terminations must be oversized. The result is a sloppy fit between the termination and the strands;
5. The individual strands are so small that they tend to wedge between opposing cutter knives rather than shearing;
6. The stranded cable often has little stiffness in compression, meaning that it cannot be reliably fed into a hole, holding device, or termination by pushing;
7. When a stranded cable is cut, many strands on the outside will fray and catch on moving equipment, feeding devices, terminations, feed holes, and the like;
8. The strands themselves are often very slick, making it difficult to control movement and to accurately measure length (because of slippage in the measurement device);
9. The stranded cable tends to flatten when passed around drive capstans and similar hardware, meaning that a constant diameter is not maintained; and
10. In order to cut the stranded cable to a consistent length, it is commonly placed under tension. When the cut occurs, a “snap back” can result. And, of course, not all the strands are cut in the same instant. When the first are cut, these snap back and the load on the remaining strands increases. Since tension is maintained approximately constant for the stranded cable as a whole, the remaining strands elongate before they are cut. The result is that the strands have different lengths, and may be displaced.
FIG. 6 shows one approach to the problem of cutting a stranded cable. Stranded cable 10 is wrapped with tape 28 (a form of localized binding mechanism) at the point where it is to be cut. Tape wrap 26 binds the cable and keeps the strands somewhat aligned. The alignment function is restricted, however, as the tape only holds the strands in the position they occupied just prior to its application. The strands may have become quite misaligned by this point, through handling and spooling of the stranded cable. Even a small bend in the cable can produce significant misalignment.
FIG. 7 shows knife 30 cutting stranded cable 10. Such a shearing action is appropriate in some circumstances. An angled guillotine type blade is also commonly used. FIG. 8 shows a possible result: Sheared surface 36 is not perpendicular to the central axis of stranded cable 10. In addition, the reader will observe that the round cross section has been deformed into an oval at sheared surface 36.
A thorough and tight taping job can minimize these deformations. With manual skill, a technician can in fact produce a perpendicular cut without substantially deforming the stranded cable. However, despite these efforts, the cable may still contain strands having unequal overall lengths. The strands may also be misaligned. Although sheared surface 36 may tend to “relax” into a more perpendicular and round shape, the discrepancies in strand length will remain. The repeatability of such manual processes is limited. Thus, it is difficult to predict the ultimate physical strengths and other properties of a cable manufactured using these prior art techniques.
In order to complete the termination process, additional steps must be performed. A termination is slipped over the tape and slid down the cable a short distance. The tape is then removed to expose the end strands. A second taping is applied at the position where the strands enter the termination. The exposed end strands are then potted into the termination.
The taping and re-taping process is obviously labor intensive. It also requires a skilled employee. FIG. 9 shows a resulting termination in cross section. The bore through the termination must be made oversized to allow for the inconsistent diameter, fiber misalignment, and lack of concentricity of the taped cable. The reader will observe that anchor 16 is not properly aligned with the centerline of the stranded cable. The assembly in FIG. 9 is obviously an undesirable result. The shortcomings of the taping example presented are also true when using other manual binding processes.